Two Chinese fighter jets crossed an unofficial dividing line in the Taiwan Strait late last month in pursuit of a US spy aircraft, according to defence sources in Taipei and Beijing.

The incident marked the first time in more than a decade that Chinese military aircraft have entered Taiwanâ€™s side of the 180km-wide strait. According to Taiwanâ€™s defence ministry, two Chinese Su-27 fighter jets briefly crossed the so-called â€œmiddle lineâ€ on June 29.

Confirmation of the close encounter comes as the US and China are trying to cool tensions in the South China Sea and safeguard a recent improvement in bilateral military relations.

Taipei, whose relations with Beijing have also been on the mend, moved to downplay the incident. â€œThis was not between Taiwan and China, but between China and the US,â€ said a senior Taiwanese defence official. â€œThe Chinese crossed the line to repel a perceived intrusion by a US reconnaissance aircraft.â€

China and Taiwan have long respected the middle line, which was drawn by the US when it signed a mutual defence treaty with the island in 1954. The line functions as a buffer zone between China and Taiwanâ€™s militaries, which still regard each other as enemies.

Chinese military aircraft have not crossed the middle line since July 1999, at a time of heightened tensions with Taiwan. That summer the Peopleâ€™s Liberation Army Airforce, which had rarely patrolled the area previously, flew hundreds of sorties over the Taiwan Strait.

China has also long objected to reconnaissance patrols of its coastline, especially since a PLA jet fighter collided with a US spy plane in April 2001 near Hainan island. The PLA pilot died and Chinese authorities detained the US crew for 11 days in a tense stand-off.

â€œThis once again shows that US military activity very close to our territory is a destabilising factor in the region,â€ said a Chinese defence source. Chinese and US military officials declined to comment.

The Pentagon declined to comment on this specific case but said US reconnaissance flights in the area were â€œfairly routineâ€ and were conducted in international airspace, as were the Chinese intercepts.

While Beijing still claims sovereignty over Taiwan and has threatened an invasion if the self-ruled island declared independence, relations between the two sides have warmed since Ma Ying-jeou was elected president in 2008.

The Taiwanese official said the islandâ€™s air force scrambled two fighters in reaction to the intrusion. He added there had not been any direct contact between the Chinese and Taiwanese planes.

According to Taiwan media, the US spy plane left the area before it could be intercepted by the PLA fighters.

Two Chinese Su-27 fighter jets crossed the unofficial centerline of the Taiwan Strait briefly on June 29 during a routine training mission, Taiwanâ€™s Ministry of National Defense said in a statement.

The act wasnâ€™t considered a "deliberate provocation," the statement said. The Chinese fighter jets were in pursuit of a U.S. spy aircraft, the Financial Times reported today, citing unidentified defense sources in Taipei and Beijing.

Taiwan has been a flashpoint in U.S.-China relations since 1949, when the defeated Nationalist Chinese government fled to the island after the Communist victory on the mainland. China claims Taiwan to be part of its territory and has more than 1,000 missiles pointed at the island nation, which it threatens to invade if it declares formal independence.

The Chinese fighter aircraft flew off soon after Taiwan scrambled jets in response, Lt. Col. Chou Wei-kun at Taiwanâ€™s Ministry of Defense said in a telephone interview today. Chou declined to comment on reports the fighters had crossed the dividing line in pursuit of a U.S. reconnaissance aircraft.

Relations across the Taiwan Strait are improving as Taiwanâ€™s President Ma Ying-jeou has worked to ease tensions by abandoning his predecessorâ€™s pro-independence stance and boosting ties with the worldâ€™s fastest-growing major economy.

The incident marked the first time in more than a decade that Chinese military aircraft have entered Taiwanâ€™s side of the strait, the Financial Times reported. The U.S. is obligated by law to help Taiwan defend itself while China considers Taiwan to be a renegade province.

A news official at Chinaâ€™s Taiwan Affairs Office of the State Council, who declined to be identified because of the agencyâ€™s rules, said she couldnâ€™t immediately comment. Calls to the office of Geng Yansheng, director general of Chinaâ€™s Ministry of National Defenseâ€™s information bureau, were not immediately answered.